1. Field of the Invention
The present invention generally relates to image forming apparatuses, and particularly relates to image forming apparatuses and control methods thereof capable of forming an image on both sides of printing material.
2. Description of the Related Art
Electrophotographic printing operations involve processes of exposing, developing, transferring, and fixing. In the exposing process, a light beam that is modulated based on image data becomes a scanning light due to a polygonal mirror, and an electrostatic latent image is formed by scanning a surface of a photosensitive drum. In the developing process, the electrostatic latent image is developed as a toner image by a development apparatus. In the transfer process, the toner image is transferred to a printing paper by a transfer roller. Then, in the fixing process, the toner image is fixed to the printing paper by applying pressure and fixing heat using a fixing unit.
In this regard, water contained in the printing paper evaporates rapidly due to the fixing heat, thereby causing a phenomenon to occur in which the printing paper contracts. This phenomenon exerts an adverse effect on double sided printing. That is, when an image that has been transferred to the front side of the printing paper undergoes fixing, the image shrinks along with the printing paper, and therefore compared to an image formed on the back side of the same printing paper, the image size is undesirably different. In other words, the magnification ratios of the front and back images formed on the printing paper deviate undesirably.
In order to address this issue, a method has been proposed (Japanese Patent Laid-Open No. 2003-241610) in which the front and back magnification ratios are made to match each other by adjusting a rotation velocity of the polygonal mirror. For example, when the image on the front side shrinks by 1% due to the fixing heat, the magnification ratio of the front side image and the magnification ratio of the back side image can be made to match each other by increasing the rotation velocity of the polygonal mirror by 1% when carrying out image forming on the back side.
Unfortunately, with the invention described in the aforementioned patent document, image forming cannot be carried out during a convergence time from the commencement of a change in the rotation velocity until the rotation velocity stabilizes. Since the convergence time is generally longer than the interval between printing papers (a so-called “time between sheets”) when continuously carrying out single sided printing, a new problem is presented in that the productivity of printing is reduced.